Nuclear magnetic resonance (NMR) chemical shift spectroscopy has been in use for a relatively long time In 1973 P. C. Lauterbur in an article published in Nature (London) 242, 89/90 disclosed the use of field gradients for determining the source location of free induction decay (FID) signals obtained in NMR experiments. The knowledge of the source of the FID signals enables the MR acquired data to be used to reconstruct interior images of the subject placed in a strong magnetic field.
It has been known that when atomic nuclei that have net magnetic moments are placed in a strong static magnetic field, the nuclei ("spins") precess about the axis of the field at the Larmor frequency given by the equation: EQU f=.gamma.Bo/2.pi.
in which:
.gamma. is a gyromagnetic ratio, constant for each NMR isotope which exhibits a net magnetic moment; PA1 Bo is the strength of the magnetic field; and PA1 .pi. is the well known constant 3.1416+.
As is well known magnetic resonance imaging (MRI) uses a relatively strong static magnetic field having a given direction which is aligned with the Z axis of a cartesian coordinate system. The strong static magnetic field causes the nuclei or "spins" of certain elements such as hydrogen to align with the field. Subsequently radio frequency pulses of sufficient amplitude and/or time duration are applied to perturb or "tip" the aligned "spins". The rotational frequency of the RF precession and the frequency of the RF pulse is the above noted Larmor frequency.
After the termination of the RF pulse the rotated spins tend to realign with the static magnetic field. The precession of the transverse component in the magnetic field generates RF signals also having a Larmor frequency. These signals are known as free induction decay (FID) signals. It is these signals that are received to provide information on the spin density of the element whose spins have been rotated by the RF pulse. The spin density information is used for imaging.
There are many different methods used for obtaining the FID signals. Among the methods and probably one of the most popular methods at the present time is the spin echo method. This method is well known and will not be elaborated on herein.
In imaging, in general, the scientists are always endeavoring to increase the spatial resolution and lower the time required to provide the image. These are contrary aims; that is decreasing the time generally may require decreasing the resolution and generally will adversely effect the signal to noise ratio. Thus, a method for decreasing the time while maintaining the same resolution and/or signal to noise ratio or a method for increasing the resolution while imaging during the same time period is highly desirable. In MR imaging, increasing the time of acquiring an image does not pose any known danger to the patient because there is no dangerous radiation being used; nonetheless, since patient comfort and throughput are important considerations effecting both the picture quality and the economics of the system, clincians and imaging scientists are always interested in decreasing the time required for acquiring images. In some cases the time saved might be used for accumulating several images of the same slice and subsequently averaging the several images to improve in the signal-to-noise ratio.
A further desired by imaging scientists is to be able to zoom during the acquisition stage. In other words, during the imaging process if a particular portion of the body shows an interesting manifestation; it is often desirable to zoom in on this maifestation and to thereby focus on the manifestation to the exclusion of other data. This was in the past often accomplished in MRI systems as a computer step after the acquisition of the data, especially if the imaging is to be accomplished within a given time frame. However, no increase of the spatial resolution can be achieved by such manipulation of the data. The second of the above referred to patent applications taught one method of zooming during the acquisition of data. Such zooming could increase the resolution of the portion of the image focused upon in a natural manner.
A problem encountered when zooming during the acquisition of data is that "aliasing" artifacts caused by undersampling may be generated unless the number of encoding cycles is increased with a proportional increase of the total acquisition time. The relationships between the field of view, the resolution and the data acquisition time are shown as follows:
The size of the volumetric aquisition matrix is: EQU n.sub.x .multidot.n.sub.y .multidot.n.sub.z
where n.sub.x, n.sub.y and n.sub.z denote the size of the matrix along the X, Y and Z axis, respectively. PA0 where l.sub.x, l.sub.y and l.sub.z are the dimension along the X,Y and Z axis, respectively. PA0 where TR is the repetition time PA0 aligning spins in a sample by positioning said sample in a homogeneous static magnetic field for obtaining NMR derived data from the sample, PA0 applying a first basic scan sequence including the steps of: PA0 irradiating said sample with a first radio frequency (RF) pulse in the presence of a first magnetic gradient so as to selectively invert the aligned spins in a planar slice of said sample, PA0 applying a modified spin echo sequence to obtain a selected strip having a sub-strip wherein the spins are 180 degrees out of phase with the spin in the rest of the selected strip, the sub-strip being a part of the planar slice, PA0 applying a second basic scan sequence including the steps of: PA0 applying the modified spin echo sequence to obtain said strip without the sub-strip, and PA0 combining the strips of said first and second modified spin echo sequences to retain only the sub-strip. PA0 x is the location along the X axis, (as an example, could also be the Y or Z axis); PA0 of is an offset frequency (added to the Larmor frequency); and PA0 .DELTA.F is the bandwidth of the RF pulse.
The volume of a voxel is V=l.sub.x .multidot.l.sub.y .multidot.l.sub.z
The field of view FOV is FOV =li * ni where i=x,y,z.
The resolution L at voxel n is: EQU L=n.sub.i /FOV.sub.i
The data acquisition time Ta is: EQU Ta=TR.multidot.n.sub.x .multidot.n.sub.y (assuming phase encoding along the X and Y axes),
It is apparent that restricting the FOV increases the resolution with a fixed acquisition matrix. Similarily restricting the FOV with a fixed resolution will decrease the acquisition time.
Localization of the volume of interest is critically important for medical diagnostic applications of magnetic resonance spectroscopy (MRS) and is useful for MRI. Selection of a cubic volume has been achieved in the prior art by a variety of techniques and system arrangements. For example, the application of RF pulse sequences comprising three consecutive tailored RF pulses, each in the presence of a different one of the three orthogonal gradients can be used to select a desired cubic volume.
The use of pulse sequences such as 90 degrees, 180 degrees and 180 degrees has been reported by R. E. Gordon et al., in a report entitled "Volume Selection for High Resolution NMR Studes" in the Proceedings of the SMRM Third Annual Meeting, 1984 at pp 272 et seq.
A pulse sequence for spatial localization using a composite pulse such as selective 45 degrees, non-selective 90 degrees and selective 45 degrees with the composite pulse applied three times, each time with one of three orthogonal gradients, has been reported in an article entitled "A Selective Volume Method for Performing Localized NMR Spectroscopy", by W. P. Aue et al. in the Journal of Magnetic Resonance, vol 56 pp 350 et seq. The method of the article is the subject of the U.S. Pat. No. 4,480,228 which was issued on Oct. 30, 1984.
A pulse sequence for spatial localization in spectroscopy using combinations of three selective 180 degree pulses and a non-selective 90 degree pulse is described in an article entitled "Image-Selected in Vivo Spectroscopy (ISIS). A New Techinque for Spatially Selective NMR Spectroscopy" by R. J. Ordidge et al. in the Journal of Magnetic Resonance, vol. 66, pp 283-294 (1986).
Yet another pulse sequence for spatial localization in spectroscopy is revealed in an article entitled "'MR Spatially Resolved Spectroscopy of Human Tissues in Situ" by P. R. Luyten et al. published in Magnetic Resonance Imaging, vol 4, pp 237-239 (1986).
Another pulse sequence for volume selection in magnetic resonance spectroscopy is explained in an article entitled "Spatial and Chemical--Shift-Encoded Excitation. SPACE, a New Technique for Volume-Selected NMR Spectroscopy" by D. M. Doddrell et al. published in the Journal of Magnetic Resonance, vol. 68, pp 367-372 (1986).
The selective 90-180-180 and the non-selective 90-180-selective 90 prior art pulse sequence procedures for spatially localizing the received NMR signals yields signals that are strongly dependent on the T2 relaxation times of the spins. This dependance on the T2 relaxation times makes it difficult to detect signals with short T2 relaxation times.
The method taught by Aue et al., i.e. the composite 45 degree--non-selective 90 degree and selective 45 degree pulse sequence requires a very high RF power and appears to be plagued by off-resonance precessional effects (see the Doddrell et al. article) that occur during the composite pulse transmission. These effects adversely effect the signal to noise ratio (SNR).
The method of Ordidge et al. is sensitive to subtraction noise and requires extremely accurate magnetic field stability to insure the exact cancellation of signals obtained from spins that are not in the volume of interest (VOI). Also, there may be a proclivity towards instrumentation problems that interfere with the efficient detection of weak signals.
The method of Doddrell et al provides relatively high power deposition and has relatively high "subtraction noise".
The method of the above mentioned second patent application has low power deposition and T2 dependence but of course uses the signals of stimulated echoes as compared with signals of a full echo.
Thus, there is still a need for an MR system that will use a full echo sequence to obtain data from selected volumes for use in spectroscopy or in imaging which will not be heavily T2 dependent, will effectively limit the RF pulse power deposition and the sensitivity to subraction noise and in addition will effectively limit the acquisition of signals to the selected volume of interest.